126 On the Stale Icsi adapted for Achrom'alic Glasses. 



the circle round it, and the division on the nonius, to coincide 

 at 10: cacii division^ therefore, is equal to the ten thousandth 

 part of an inch. 



Apphing this little instrument to the eye-glass of a telescope, 

 when adjusted to distinct vision at any distant object, and turn- 

 ing the micrometer head, tlie emergent pencil will begin to se- 

 parate ; and when the extrenje edges are brought into contact, 

 the number of divisions will show the diameter of it in thou- 

 sandths of an inch ; then reduce the diameter of the object-glass 

 into thousands, and divide that sum by the diameter of the pen- 

 cil, the quotient will be the real magnifying power. But as it is 

 requisite for the emergent pencil of rays to be in the focus of the 

 divided glass, a thin transparent piece of ivorj, precisely one- 

 tenth of an inch in diameter, is set in the sliding cover, to adjust 

 for that distance, which must always be done before it can be 

 used with accuracy. 



When this transparent piece of ivory is brought over the hole 

 in the cover of the dynameter, and appears perfectly round, the 

 nonius will then be at 0, and is properly adjusted. Five revolu- 

 tions of the micrometer screw will make a complete separation 

 of the diameter of its aperture, which is one-tenth of an inch : 

 and when the opposite sides are brought into contact, the nonius 

 will coincide at the fifth division of it, which is five two-hun- 

 dredths of an inch ; thus dividing each tenth of an inch into a 

 thousand equal parts. Another method of discovering the mag- 

 nifying power, is to set the telescope in such a position opposite 

 the sun, that the rays of light may fall perpendicularly on the 

 object-glass ; and the pencil of rays may be received on a piece 

 of paper, and its diameter measured: then, as the diameter of 

 the pencil of rays is to that of the object-glass, so is the magni- 

 fying power of the telescope. Or, thirdly, a thin piece of mother 

 of pearl, with a very acute angle two inches long marked thereon, 

 and only one-tenth of an inch at its base marked thereon ; the 

 length being divided into ten equal divisions, making a visible 

 line to each division, with a figure over it, — these divisons will 

 express or show the hundredths of an inch: apply this scale to 

 the eye-tube of the telescope, observe where the emergent pencil 

 of rays fills up a certain space at or near any of the divisions ; 

 multiply the diameter of the object-glass into hundredths on the 

 scale, and the (juotient will be the magnifying power. 



Before any of these methods of finding the magnifying power 

 be made use of, remember to look through the tube, and observe 

 carefully if some of the object-glass be not cut off, and part of 

 the original pencil intercepted by the stops in the tube, &c. This 

 is a very common trick, and will render your calculation on the 

 whole aperture erroneous ; for in all cases the magnifying power 



of 



